you think someone merged the account to a battle.net few months ago and played it and lvled it to lvl 90 , then transfed the char away ,then he unmerged the account again, and then i sold those accounts to you ? In that case , you can check the char transfer page history if there is any transfer history .
And all of us know unmerged accounts cant log in without a battle.net email ,and after those account merge , you cant unmerge it from one battle,net email then merge to another battle.net email. But seems you did it , and not only one but 3 of them .
I have also asked other sellers in the world plus i have sold those accounts over 2 years , never heard this happen , Maybe you got something wrong .